Since there are many 'guardian of fire' Naruto fanfictions, the main characters can vary greatly. It could still have Naruto as a main character, along with the new 'guardian of fire' character, who could be an original creation for the fanfiction.
Most likely Naruto would be one of the main characters. After all, it's in the Naruto universe. And then there would be the fairy guardians. They could be named characters with their own distinct personalities, like a headstrong fairy guardian or a wise old fairy guardian.
Obviously Naruto would be one of the main characters since he is the guardian. And there would probably be some Pokémon characters like Pikachu or Charizard that he interacts with.
Other Naruto characters like Sasuke or Sakura might also be important. Sasuke could be involved in Naruto's new role as a 'guardian' either as a supporter or a skeptic. Sakura, with her medical skills, could play a role in helping Naruto in his 'guardian' - related adventures, perhaps by healing those affected by new threats that come with the combination of the two universes.
Another possible main character could be Erza from Fairy Tail. She is a strong and respected member of the guild. Her leadership skills and combat abilities could be crucial in the story. She might initially be skeptical of Naruto but then come to respect him as he proves his worth as a guardian. And Naruto himself would have his own set of friends and allies from his world that could potentially make appearances or be mentioned, like Sasuke or Kakashi, even if they are not directly in the Fairy Tail world, their influence on Naruto could still play a part in the story.
I'm not sure specifically as there could be many different 'Guardian of Fire' Naruto fanfictions. Generally, it might be about a character who becomes the guardian of a powerful fire - related element in the Naruto universe. It could involve new adventures, battles, and character developments that are not part of the original Naruto story.
Obviously, Naruto is likely to be one of the main characters since it's in the title. There might also be characters like Sasuke, who could be Naruto's rival or ally in the context of this fanfiction related to the god of fire. Sakura could also be there, perhaps as a healer or someone who helps Naruto understand his new powers.
Naruto is surely the central character in this fanfiction. Besides, characters like Kakashi could be involved. He might be Naruto's mentor in controlling and understanding his new 'king of fire' powers. And villains from the Naruto universe might also appear. They could be envious of Naruto's power and try to take it away or use it for their own evil purposes. So, a variety of characters from Naruto's world could be main characters in this fanfiction depending on the plot.
It's possible that the main characters could also include some of the antagonists from the original series. Maybe they are behind the cause of the fire and ash, or they take advantage of the chaos to further their own goals. Naruto and his allies would then have to face these enemies while also dealing with the desolate environment.
It could be the descendants of the original Naruto characters. For example, Naruto's son might be one of the main characters, carrying on his father's legacy and the will of fire.
In some fanfictions, the main character could be a new - generation Guardian of the Mist who is related to an important figure from the village's past. This connection drives their actions and decisions. There might also be a group of comrades within the Mist village who support the Guardian in their duties, and they could be important secondary characters.